Components of switching devices are subject to very stringent requirements. In general the electrical components are provided with a sheathing which consists of an electrically insulating synthetic resin. In addition to the required dielectric characteristics of the sheathing, mechanical characteristics such as impact resistance and the inclination to form cracks are of importance.
A critical factor for a sheathing resin for high performance electrical components such as vacuum switch gears or transformers working in a voltage range up to about 40 kV is the resistance against crack formation during temperature variation.
An indication for the thermal cycle crack resistance is the so-called crack index value (RI).
In order to avoid cracks in the sheathing material the prior art proposes that the vacuum chambers of switch gears and other parts are installed in the dielectric moldings and are encapsulated into the load-bearing enclosure composed of an epoxy resin. In order to prevent crack formation the molding composition contains filling powder such as quartz powder or synthetic silica powder.
However, the crack index value (RI) achieved by this method is not sufficient for high performance electrical components.
Furthermore, electrical components have been encapsulated in silicone or polyurethane or a “soft” casting resin to have a kind of buffer that withstands the stresses due to mismatch of thermal expansions.
The encapsulation technique means that the vacuum switching chamber and the inserted parts must be cushioned, for mechanical reasons, by means of an elastomer material before the introduction into the epoxy resin. The requirements for this material are:    high dielectric strength and adequate elasticity to absorb thermal stresses and mechanical stresses.
The elastomeric cushioning absorbs stresses which occur in the encapsulated components during the operation due to a mismatch of different thermal expansions of the encapsulating material and the component to be encapsulated.
However, this technology requires at least two process steps which is less economic and, furthermore, time consuming.
Thus, a molding process is preferred wherein the epoxy resin is directly applied to the vacuum chamber of a switch gear and which does not require an additional cushioning.
WO-A1-2004/090913 describes a method for the production of moldings for switching devices for low-voltage, medium-voltage and high-voltage applications wherein a mixture of glass balls with a predetermined distribution of diameters of size Dx is introduced into the encapsulation compound thus creating direct encapsulation of components. However, the method for the direct overmolding of switching devices disclosed in WO-A1-2004/090913 suffers from high costs of the hollow spheres and, furthermore, the hollow spheres may be partly mechanically destroyed during the mixing process which reduces the efficiency.
Further, it is disclosed that additional fillers such as amorphous silica and wollastonite may be used. However, the application does not teach that a certain combination of these two fillers would be suitable as filler for epoxy systems to be used for direct overmolding of vacuum switch gears.
EP-A2-1 176 171 discloses a dielectric material for direct overmolding of switch gears comprising a heat curable matrix resin containing epoxy resin and modified acid anhydride as well as inorganic particles and rubber particles having a core/shell structure. However, the combination of inorganic particles together with rubber particles having a core/shell structure is very expensive.
U.S. Pat. No. 6,638,567 B1 discloses a curable composition comprising:    (a) a cycloaliphatic epoxy resin that is liquid at room temperature and, suspended therein, a core/shell polymer;    (b) a polycarboxylic anhydride; and    (c) two different fillers, (c1) and (c2), wherein (c1) is a filler that is able to release water as the temperature rises above room temperature; (c2) is a reinforcing material; the total proportion of fillers (c1) and (c2) is from 58 to 73% by weight based on the total amount of components (a), (b), (c1) and (c2) in the composition; and the ratio by weight of the fillers (c1):(c2) is in the range from 1:3 to 1:1. The curable composition can be used for the direct overmolding of vacuum switch gears and other high performance electrical components. However, core-shell polymer toughened systems are very expensive.